Making idle stronger
Thread Starter
Teching In
Joined: Jun 2022
Posts: 30
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Making idle stronger Howdy, I have an LQ9 6.0 with a texas speed torquer II and a Holley 650 DP on top, with a 5 speed manual trans.
The car can't stay alive at idle with the choke off until it hits exactly 180*, then it'll stay alive when coming to a stop, but not before dipping and hunting for RPM's a bit. Curb idle speed screw is set at about 900
I've heard of adding timing to "strengthen" the idle, but I haven't been able to figure out how to do so.
My timing is as follows:
RPM Timing
0 20*
500 32*
1250 24*
1900 29*
3000 35*
The car can't stay alive at idle with the choke off until it hits exactly 180*, then it'll stay alive when coming to a stop, but not before dipping and hunting for RPM's a bit. Curb idle speed screw is set at about 900
I've heard of adding timing to "strengthen" the idle, but I haven't been able to figure out how to do so.
My timing is as follows:
RPM Timing
0 20*
500 32*
1250 24*
1900 29*
3000 35*
TECH Enthusiast
Joined: Sep 2019
Posts: 532
Likes: 630
Howdy, I have an LQ9 6.0 with a texas speed torquer II and a Holley 650 DP on top, with a 5 speed manual trans.
The car can't stay alive at idle with the choke off until it hits exactly 180*, then it'll stay alive when coming to a stop, but not before dipping and hunting for RPM's a bit. Curb idle speed screw is set at about 900
I've heard of adding timing to "strengthen" the idle, but I haven't been able to figure out how to do so.
My timing is as follows:
RPM Timing
0 20*
500 32*
1250 24*
1900 29*
3000 35*
The car can't stay alive at idle with the choke off until it hits exactly 180*, then it'll stay alive when coming to a stop, but not before dipping and hunting for RPM's a bit. Curb idle speed screw is set at about 900
I've heard of adding timing to "strengthen" the idle, but I haven't been able to figure out how to do so.
My timing is as follows:
RPM Timing
0 20*
500 32*
1250 24*
1900 29*
3000 35*
Thread Starter
Teching In
Joined: Jun 2022
Posts: 30
Likes: 2
Thread Starter
Teching In
Joined: Jun 2022
Posts: 30
Likes: 2
If I had to guess it's probably lean. If you don't have an air/fuel gauge I would recommend investing in a vacuum gauge and adjust your carb (engine warm) until you get the idle RPM you are looking for at the highest possible vacuum, and that will be 90% as good as a perfect idle mixture.
I’m curious to hear more feedback on my timing.
edit: thanks for replying btw
TECH Resident
Joined: Jun 2011
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First of all 20* at 0 is going to put a lot of load on the starter, especially when the engine is hot. 32* is way too much @500rpm. The whole map is strange.
I don't know how many points your system allows (it would be nicer if there was more points for better resolution), but if it's only 5, try this as a base map:
16* @ 500rpm
20* @ 750rpm
25* @ 1250rpm
30* @ 2800rpm
35* @ 6000rpm
And then readjust your idle circuit on the carb. It should crank easier, stabilize the idle, and hopefully make more power everywhere. You can always adjust from there to optimize.
I don't know how many points your system allows (it would be nicer if there was more points for better resolution), but if it's only 5, try this as a base map:
16* @ 500rpm
20* @ 750rpm
25* @ 1250rpm
30* @ 2800rpm
35* @ 6000rpm
And then readjust your idle circuit on the carb. It should crank easier, stabilize the idle, and hopefully make more power everywhere. You can always adjust from there to optimize.
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Thread Starter
Teching In
Joined: Jun 2022
Posts: 30
Likes: 2
First of all 20* at 0 is going to put a lot of load on the starter, especially when the engine is hot. 32* is way too much @500rpm. The whole map is strange.
I don't know how many points your system allows (it would be nicer if there was more points for better resolution), but if it's only 5, try this as a base map:
16* @ 500rpm
20* @ 750rpm
25* @ 1250rpm
30* @ 2800rpm
35* @ 6000rpm
And then readjust your idle circuit on the carb. It should crank easier, stabilize the idle, and hopefully make more power everywhere. You can always adjust from there to optimize.
I don't know how many points your system allows (it would be nicer if there was more points for better resolution), but if it's only 5, try this as a base map:
16* @ 500rpm
20* @ 750rpm
25* @ 1250rpm
30* @ 2800rpm
35* @ 6000rpm
And then readjust your idle circuit on the carb. It should crank easier, stabilize the idle, and hopefully make more power everywhere. You can always adjust from there to optimize.
The timing box offers unlimited points I believe - but I'm going to try this and also evaluate the idle mixture with my vac gauge
Thanks again!
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Verdad Gallardo TECH Enthusiast
Joined: Sep 2019
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It's not an LS but after installing the O2 sensors in my truck I adjusted idle timing to give me the max RPM at a given carb setting, which is after getting the carb tuned where it read just a hair rich like 13.5 or 14:1 as you described. It doesn't have a choke, but it can start and run in freezing weather.
TECH Enthusiast
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Posts: 532
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Before when I had it tuned by "feel" and smell it was a bit lean at idle, rich in the cruise, lean on tip-in of the throttle, and a hair lean at WOT. A few jets, metering rods, and springs later it runs right on the dot, and gained about 1.5mpg with the lean cruise. I can watch the AFR go from 14.5-15.5 in the light cruise to 13.5 when the metering rod jumps and around 13:1 on the secondary opening at full throttle.
The 650 double pumper may be a bit more difficult to get just so than the Carter, though.
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13:1 is okay. Maybe just a hair leaner than max power but still safe. If I had more compression or wanted to fight pinging I would consider a slightly richer secondary jet. The spark plugs look good, though.
Thread Starter
Teching In
Joined: Jun 2022
Posts: 30
Likes: 2
If I had to guess it's probably lean. If you don't have an air/fuel gauge I would recommend investing in a vacuum gauge and adjust your carb (engine warm) until you get the idle RPM you are looking for at the highest possible vacuum, and that will be 90% as good as a perfect idle mixture.
Thread Starter
Teching In
Joined: Jun 2022
Posts: 30
Likes: 2
Yes, very much so. Better to start a little rich and turn them in anyway. That's why you typically hear start 2 turns out. That'll be very fat in most cases. It won't be the same AFR for every engine and every carburetor combination, but it's a good starting reference in 90% of cases if everything else is right.
Couple pieces of advice.
1) When you seat the screws, use 2 fingers to turn the screwdriver. It's easy to damage the needles with too much force. Then you lose a little bit of fine adjustment.
2) Adjust the mixture with the filter assembly in place. The filter housing directs the flow and the filter itself can affect how much enters even during idle. I've seen nearly a 1point difference in AFR between no filter assembly versus filter assembly on top.
3) If you find that turning the screws produces no effect, then the idle speed set screw is probably so far down that the throttle plates have uncovered the transition slots and that's feeding too much fuel. You could then verify this by disconnecting the small vacuum port and then turn the idle speed back down and see if that allows you to adjust the mixture.
Couple pieces of advice.
1) When you seat the screws, use 2 fingers to turn the screwdriver. It's easy to damage the needles with too much force. Then you lose a little bit of fine adjustment.
2) Adjust the mixture with the filter assembly in place. The filter housing directs the flow and the filter itself can affect how much enters even during idle. I've seen nearly a 1point difference in AFR between no filter assembly versus filter assembly on top.
3) If you find that turning the screws produces no effect, then the idle speed set screw is probably so far down that the throttle plates have uncovered the transition slots and that's feeding too much fuel. You could then verify this by disconnecting the small vacuum port and then turn the idle speed back down and see if that allows you to adjust the mixture.
I chased down a misfire for months that caused me to remove the carb about half a dozen times to confirm the T-slot was within spec. So I'm plenty familiar with "resetting" the idle mixture and throttle screws hahaha. But I've found with carbs that you can't have enough reminders and tips. Even though I know the routines by heart, I still research and consult for advice to see if I missed anything.
Case in point, I set the mixture screws to 1.5 turns out. Maybe I'll bump that up to 2 turns from seated.
Hopefully will test the car tomorrow - thanks again for the input.
Edit: bumped it up to 2 turns out from seated, hopefully that'll simplify tuning a bit
Last edited by 5thandmuscle; Oct 2, 2023 at 08:39 PM.